Fluid Mechanics Applications/A02: Delta Wing Aircraft Aerodynamics

Research in supersonic aerodynamics began in the period of II World War i.e. the 1940's. This research was howerver limited to conventional wings. Howerver with the introduction of Linear theory resaerch came to be directed towards the application of linear theory to designing aerodynamic vehicles to achieve a single purpose-efficient supersonic flight.The initial result of this research was the identification of the delta or triangular wing planform as the most likely candidate for achieving supersonic flight. Moreover, it was found out experimentally that the empirical relations for delta wings could be extended to other wing platforms as well, like arrow and diamond shaped wings , through the use of geometrical and flow relation parameters.

What are Delta Wings?

The delta wing is a wing planform in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta (Δ).It is different from the wings we see in commercial planes, they being swept wings.

Zero Lift Wave Drag

According to supersonic linear theory the zerolift wave-drag (C_D,W) in a delta wing varies as the airfoil thickness-to-chord ratio squared (τ²) and as the wing aspect ratio (A) for a given airfoil profile.

Why Delta Wings

The primary advantage of the delta wing is that, with a large enough angle of rearward sweep, the wing’s leading edge will not contact the shock wave boundary formed at the nose of the fuselage as the speed of the aircraft approaches and exceeds transonic to supersonic speed. The rearward sweep angle vastly lowers the airspeed normal to the leading edge of the wing, thereby allowing the aircraft to fly at high subsonic, transonic, or supersonic speed, while the over wing speed of the lifting air is kept to less than the speed of sound. Some other advantages of Delta wings are:-

• Rise in Transsonic drag is gradual and peak drag in supersonic flight is reduced—Lift is spread over the broader chord

--Senstivity of Drag to Mach Number is reduced greatly
--Satisfactory cross sectional area can be achieved very easily

• Change in lift component C_L and lift curve slope C_Lα with the value of Mach Number M is gradual.
• A very high stall angle is achieved by increasing the angle of attack, as a flow energising vortex is generated by the leading edge of the wing at high angles of attack.
• It allows for light wings with high bending and torsional stiffness.

--More volume for fuel and gear is available in thicker wings.
--Fluttering and aileron reversal problems are wholly eliminated.

• Acceptable level of maneuverability and ease of handlingis achieved because of low wing loading.
• Wingspan is small and hence does not require folding.
• Available area for external storage is large enough.
• It is easy ,simple and relatively inexpensive to manufacture.
• The addition of canards to the aircraft increases the overall lift, enabling the aircraft to perform extreme maneuvers and at the same time improving the low speed handling and lowering the landing speed

Disadvantages of Delta Wings

• Very high landing speeds and bad field performance by tailless deltas—Higher angle of attack is required for low lift-curve slope.

--Angle of attack is limited by tail clearance.
--It is unable to trim out nose downpitching movement caused by the flaps.

• Lift induced drag is very high in subsonic conditions.

--Thrust required is very high
--At high angles of attack there is trimmed loss of lift because of the downloading trailing edge controls.

• Wing loading must be low—Despite a low C_Lα ,L_α is high because of low gust response.

-- Maneuverability is compromised by high wing loading

• Maneuverability at supersonic speeds is restricted.

--Effectiveness of elevons i.e.the trailing edge controls is reduced. --Large absolute shift in a.c. (needs to be trimmed and may demand c.g. shift)

• At low speed C_lβ is high—Desired relationship between lateral and directional stability is disturbed by the large sweep (of leading edge) and the high angle of attack

--Exaggeration of Dutch roll occurs.
--There is a requirement of a low wing and yaw dampers.

• If there is no horizontal tail , there is reduction in pitch damping.

--It runs a risk of oscillations induced by the pitch.
--Installation of pitch dampers is required.
--To avoid deep stall at high angle of attack , a low mounted horizontal tail is used.

Improvements in Delta Wings

Due to the various disadvantages of delta wings considered there was a need for improvement which led to the development of compound delta wings. In compound delta wings a more highly swept delta wing is added in front of the main delta wing. This creates a controlled vortex and further reduces the drag.